Process of color-correcting titania-bearing porcelain enamels



Patented July 11, 1950 PROCESS OF COLOR-CORRECTING TI- TANIA-BEARING PORCELAIN ENAMELS Donald R. Goetchius, Avon, and Edwin J. Kelly, Cleveland, Ohio, assignors to Ferro Enamel Corporation, Cleveland, Ohio, a corporation of Ohio No Drawing. Application August 15, 1947, Serial No. 768,930

This invention relates as indicated to titanium opacified porcelain enamels and also to the provision of a titanium-bearing opacifier which may be used in porcelain enamels and when so used, will give uniform results as to color effect even though the titanium may be derived from different naturally occurring sources and with different amounts of various impurities.

The use of titanium as an opacifler in porcelain enamels has been recognized as desirable for many years; It is more expensive than many of the more commonly used opacifying materials but it is preferable over the latter because of its high covering power or clouding effect and also because by its use a highly acid-resistant porcelain enamel may be produced.

Up to the time of our invention titanium dioxide was not used for opacifying purposes to any substantial extent in commercial production because of the difiiculty of color-matching porce- 5 Claims. (Cl. 106-312) lain-enameled articles produced from difierent batches of commercially available titanium dioxide. The prior workers in the art found that whereas one batch of commercially available titanium dioxide when added to porcelain'enamel produced a highly reflective product having a light cream color, another batch of the titanium dioxide added in the same amounts, in the same enamel, and all other conditions maintained the same would produce a product having a light bluish cast. Not only were these variations in color quite different but they were also entirely unpredictable. Some workers in the art believed that the color changes were due to minor amounts of iron present in the commercially produced titanium dioxide and oxidation or reduction of the melt. Improvements in refining technique resulting in the elimination of most of the iron did not, however, solve the problem. A report on an extensive investigation of the yellow discoloration of titanium-bearing enamels will be found in report No. BD156 dated September 26, 1938 and published by the British Titan Products Company Limited. The workers who conducted that investigation suggested as the only remedy, asidefrom the reduction of i the ironcontent to as low a percentage as possible, the use of small additions of antimony oxide. Aside from the fact that the addition of antimony oxide does not solve the problem and causes a reduction in opacity, the use of antimony is frowned upon and in certain instances prohibited in enamelswhich come incontact with food. In a subsequent report No.-BD -228 bythe same .companyunder'the authorship of I S. G. Tinsley, other suggestions were made as a possible solution to the problem. However, none of such suggestions were found to be useful and effective in producing the same color effect when different batches of commercially produced titanium dioxide were employed.

As a result of extensive investigation of the composition of the commercially available batches of titaniumdioxide and with particular regard to those impurities which occur in trace quantities and which are usually reported as traces in the analyses of commercially available batches of titanium dioxide, we have discovered that in the impurities lies the secret to the control of uniform color effect in porcelain enamels which are opacified by the use of titanium dioxide.

It is a principal object of our invention, therefore, to provide a method whereby the color effectiveness of commercially produced titanium dioxide may be standardized so that even though the normal color effectiveness of different batches of commercially available titanium dioxide would be quite different nevertheless the same color method of rendering different batches of commercially produced titanium dioxide, which may or may not contain minor amounts of at least one of the elements of tantalum, columbium and tungsten as impurities, uniform as to color effect when used as an opacifier for porcelain enamel by being smelted with the raw batch in producing the frit, which comprises adding to the different batches'respectively such amounts of at least one of the elements tantalum, columbium and tungsten so that the total percentage of all of such elements in each of the batches is substantially the same. 1

The broad method of our invention is particularly applicable to different batches of commercially produced titanium dioxide which are substantially free of chromium.

Many manufacturers of porcelain enamel products require that the final product be essentially white with a slight bluish cast. It was found that that result could be secured by the use of certain batches of commercially available titanium dioxide whereas other batches of apparently the same composition produced an end product which had a decidedly cream tinge. As previously indicated, we therefore investigated the difference between the compositionoi those batches with particular regard to those elements Whose presence is ordinarily not reported but which when requested in an analysis are usually reported as traces. By careful spectographic quantative analysis for these trace impurities it was found that the aforesaid difference in color efiectiveness of the diiierent batches ofthe titanium dioxide was due to differences in the total amount within which the elements tantalum, columbium and tungsten were present. It was found that when the total amountof these elements was present to the extent of at least 0.2 per cent based on the total amount of titanium dioxide present, the porcelain enamel opacified by such titanium dioxide always had the desired bluish tinge. It was also found that when the total amount of these three elements was changed, the intensity of such bluish tint varied in almost direct proportion. Thus when the three named elements were present in a total amount of about 0.1 per cent the porcelain enamel end product had a substantially neutral white appearance as determined by carefully controlled reflectance tests using different colored filters. It was also found that when these three elements were either absent entirely or present in amount much less than'OJ-per cent, the enamel containing such opacifier had a'decidedly creamy tinge.

Further investigation ofthese commercially available samples having diiferent' amounts of the three named elements present showed that when the total of the same was only 0.1 percent and the enamel opaoified thereby accordingly had a neutral white color, the desired bluish tinge could be imparted to the enamel by the addition of 0.1 per cent of any of the three elements. In one instance when thetitanium dioxide contained a total of'0.1 per cent-of tantalum, columbium and no tungsten, the desired bluish tinge was secured by the'additionof 10.1 per cent of tungsten based on the amount of titanium dioxide.

Since-the advent of our discover'yas explained above, we have been able to produce uniform results'as to color by using a wide variety of commercially available batches of titanium dioxide. When we received a new batch of titanium dioxide, prior to the time it is used weiirst analyze the same quantatively by the spectographic method and determine within 0.01. per cent the amount of colombium', tantalum and tungsten present. We then blend with such sample an amount of tungsten to bring the total of these three elements to a predetermined amount. For our purposes, we havestandarclize'd on a total quantity of thethree elements of 0.2 per cent based on the amount of titanium dioxide present for the production of an enamel which has a slight bluish tinge and on a total amount of 0.1 per cent or" the three named elements for an enamel which has a substantially neutral white color.

When the spectographic analysis shows that the sample of titanium dioxide contains'atot'al of less than .02 per cent of columbium, tantalm and tungsten then if there are no other coloring impurities present, the enamel opacified thereby will have a light cream tinge. The color of such an enamel may be adjusted to a neutral white by the addition of about 0.1 per cent of tungsten based on the amount of titanium dioxide, and if a definite bluish tinge is desired it may be secured by the use of about 0.2 per cent of tungsten based on the amount of titanium dioxide.

Amounts of the three critical elements in excess of atot'al or 0.2 per cent will give a bluish tinge which is toopronounced to be acceptable as a white enamel. Amounts of the three elements in excess of 0.2 per cent may be used, however, where for example the enamel com- "position is such that it has an unusually pronounced creamy tinge or when a more decided bluish tinge is desired.

Tungsten in the formof the oxide as the corrective additionagerit is usually used because, of the three critical elements involved, it is the most readily available and can be secured at the lowest cost. The critical elements may be added in the form of any of the compounds thereof which will be soluble and be retained in the fused glass and will not add other harmful elements such as chromium or iron .for example. Instead of adding an auxiliary amount of one of the three named elements such as tungsten the same results may be secured, of course, by the admixture of difierent batches of titanium dioxide which have diiTerent-total amounts of the three critical "elements naturally present therein. This expedient'isparticularly desirable for use in connection with batches of titanium dioxide which,-asreceived containmore than the optimum predetermined total amount of the three elements. Such a batch may be mixed with another batch in which the total amount of the three critical elements is below the optimum figure and by controlling the relative amounts of the two batches a composite product may be produced in which the total amount within which the three elements are present is at the desired value.

In our investigation of the amounts within which the various traoe;elements-arerpresent in commercial batches of titanium' dioxide, we also discovered that thebatches originally received by us contained varying and substantial quantities of chromium. It was also found that that'elemont even in small amounts =had'a decidedinfiuence-on the colorei fectof the'tita-nium dioxide. The removal of chromium is not difiicult and may be readily rcducedgto the-desired minimum in the commercial Iprocess for ,pro-

ducing ceramic grade titaniumdio'xide. Therefore it is practical 'to obta'i'necommercially titanium dioxide which 'is sui'thcien'tly low in chromium content.

The commercially produced titanium 'dioxide which has been substantially freed 'ofch'romium and which contains the properly adjusted 'total amounts of columbium, tantalum' and tungsten, as above explained, rmay be used astan opacifier either as a smelter'addition or 'as amill addition. Its use as a 'smelteraddition is particularly desirable because of its effect 'in' producing an enamel which is highly opaque: and acid resistant. As a smelter addition it may beemployed inan amount of from about 5 :per cent to about 20-per cent of 'thetotalweight of the rawmix. The following'is a range'of percentages within which the various components of the raw mix Dehydrated borax -Q Zinc oxide may be employdlnproducing a seriesofnamL els Io'pacified with the improved opacifier ofour nv tion. w g

In the followingtable is given the raw batch composition of one enamel falling 7 .within, the above range and which will be found to'bea particularly-useful enamel. c a t Table Feldspar Soda ash Potash nitrate Sod. sil. fiuoride Titanium oxide (improved opacifier of this invention) 18 Silica 38 When the raw mix of Table B is smelted, the

calculated oxide content. of the frit produced therefrom will be approximately as followszx -T Table C NazO. L '8L38 A1203 1.62 SiOz 45.80 B203 11.65 N20 5.10 A1203 1.62 ZnO 2.00 F 3.06 T102 18.00

The enamels of Tables B and should be smelted at a temperature of approximately 2250* 'F. The frit produced therefrom may be employed with the mill additions given in Table D.

Table D Frit "pounds" 100 Enamelers clay do i Titanium oxide (commercial ceramic grade) do.. 1 Potash carbonate ounces 8 Water pounds 38 The mill charge given in Table D above, milled to a fineness such that 1 to 2 per cent will remain on a 200 mesh screen, may be advantageously employed at a weight of application of about grams to 30 grams per square foot applied over a regular ground coat. A single coat of grams per square foot will be found to produce a satisfactory finish for articles such as stove parts, refrigerator parts and kitchenware. A plurality of coats of this same enamel or as a finish coat over an intermediate coat'of another type of white enamel may be employed for the production of a slightly better finish such as may be required for example on; articles such as sanitary ware. r

The-frit produced by the use of our improved opacifier as a smelter addition is characteristically transparent like all of the titanium bearing enamels which aresuddenly quenched from ani'elevated temperature, 1. e. the titaniumdiox- .idexis in solution in the glass matrix. Upon fusing :the enamel the titanium dioxide will crystallize mostlyin the anatase form. Such crystallization usually occurs during the firing of the enamel in its application to the ware. By the use of our improved opacifier in which the total amount of columbium, tantalum and tungsten is equal to about..2 per cent of the titanium dioxide content, Wehave been able to secure in a single coat of application; an enamel of a desirable blue-white color-with a reflectance as high as 78 per cent.

The mechanics by which this balance between the amount of titanium dioxide and the total amount of tungsten, tantalum, and columbium present produces the stated and desired effect-is not fully understood. It is known that the efiect which is thus-secured is not the same as that secured byjthe use of a conventional coloring oxiide such as cobalt oxide. There is some evidence that the three critical trace elements have a strong effect on the crystal growth of titanium dioxide as it crystallizes in the glass matrix during the'firing of the enamel on the ware. We are convinced thatthe three critical trace elements, tantalum, columbium and tungsten, when present in an amount of about .1 to .2 per cent very substantially reduce the crystal size of the titanium dioxide as it precipitates in the enamel on ,firing. The decrease in crystal size results in a slight loss of opacity and a change of color to have astrong influence in reducing the state of oxidation of the titanium dioxide to one of its blue oxides. These properties, i. e. of controlling crystal size and the state of oxidation of the titanium, are ones which are not possessed by any of the conventional coloring oxides such as cobalt oxide. An attempt to control the color of the fired enamel by the addition of a coloring oxide either to the melt or to the mill addition to give a bluish cast will usually result in off-shade greenish whites. An excessive lowering of reflectance will also occur.

Throughout the appended claims we have referred to different batches of commercially produced titantium dioxide. There are a large number of patents which disclose various ways of producing titanium dioxide. A notable example is Patent No. 1,932,087 issued to Richter, October 24, 1933. Another example of a somewhat different method is given in the patent to Rossi et al. No. 1,106,406 dated August 11, 1914. Regardless of the method employed, most commercially available batches of titanium dioxide will amines tures stated in any ofthe following claims or the equivalent of such be employed.

We therefore particularly point out iandldistinctly claim as our invention:

'1."'I he methodof rendering uniformfa's tocolor, difierent hatches of porcelain enamel Prespectively opa'cified-hy having s'm'elted'therein different batches of commercially produced titanium dioxide, which 'comprises'lfirst determining :the total -amount of tantalum, col umbium an'dtungsten present as impurities in'said' titanium dloxide, and then adding to said titanium dioxide at leaston'e of the e1ements selected Efrem the group consisting of tantalum, columbium and tungsten-so thatthe total percentage of all said elements'in each oi the titanium 'd-ioxi'de batches fist-rem about 0.1% to ahout ;2'%, saiditota1 .p'ercen tage of-all s'aid' elements in each of the titan- "ium dioxide batches having substantially the same Value.

2. The method of rendering uniform as to 'color,-different batches or porcelain enamel respectively opaeifi'ed by having smelted therein difierent batches of commercially produced titanium dioxide, which comprises first determinin the total amount of tantalum, colu mloiuiri and tungsten present as impurities in said titanium dioxide, and thenadc l-in'g tosaid titanium dioxide at least one-of theelements selected irom 'the' group consisting of tantalum, columbium and tungsten so that the total --p'ercentage of all :said elements in each of v the titanium dioxideloatches is abo'ut 0.2

'3. The method of rendering uniform as to color, different "batches of "porcelain enamel resp'ectively opac-ified by -ha v-ing smelted therein different batches of commercially produce'd titanium dioxide which are-=substantially ireeof chromium, which comprises first determining the total :amount oithe elements tantalum, columbium, and tungsten present as impurities in said titanium dioxide, and then adding tosaid'titan- -ium dioxide-atleast 'one of the elements selected from the group consisting of tantalum, columbium and tungsten so that the total percentage of all said elements in'e'ach of the titanium dioxide batches is about 0.1%.

4 The method of rendering uniform as to color, difierentbatches oi "porcelain enamel respectively 'onacified "by havingsmelted therein di'fierenthatches of c'ommercia'llypro'duced titan- 'ium dioxide 'which are substantially free of chromium;whi-ch comprises first determining the total'amnunt of the elements tantalum, columbiumand tungsten present as impurities in said titanium di0xi'd'e,'and then adding to said titanlium dioxide'anamount'oi"tungsten to bring the total percentage of all said elements in'each of the titaniumnioxidebatches'to'about 0.2%.

5. The "method of rendering uniform as to color, different hatches "of porcelain enamel respectively opacified by having smelted therein differentbatches of commercially produced titanium dioxide, which comprises first determining the total amount of the elements tantalum, columbium, and tungsten present as impurities in said titanium dioxide, and then adding to said titanium'dioxide an amount of tungsten to bring the total percentage" of all'said elements 'in "each "of the'titanium'dioxide -hatches 'toabout "0.1%.

DONALD R'GOETCHIUS.

J. KELLY.

REFERENCES CITED The idllow'ing references are of record in the file of this patent:

UNITED STATES PATENTS Number :Name Date 1,249.;937 Eyer -Dec. 11, 1917 2,102,627 Kinzie Dec. 21, 1937 2,116,623 Flint a May 10, 1938 2,200,373 Oppegaard et al. May 14, 1940 

1. THE METHOD OF RENDERING UNIFORM AS TO COLOR, DIFFERENT BATCHES OF PROCELAIN ENAMEL RESPECTIVELY OPACIFIED BY HAVING SMELTED THEREIN DIFFERENT BATCHES OF COMMERCIALLY PRODUCED TITANIUM DIOXIDE, WHICH COMPRISES FIRST DETERMINING THE TOTAL AMOUNT OF TANTALUM, COLUMBIUM AND TUNGSTEN PRESENT AS IMPURITIES IN SAID TITANIUM DIOXIDE, AND THEN ADDING TO SAID TITANIUM DIOXIDE AT LEAST ONE OF THE ELEMENTS SELECTED FROM THE GROUP CONSISTING OF TANTALUM, COLUMBIUM AND TUNGSTEN SO THAT THE TOTAL PERCENTAGE OF ALL SAID ELEMENTS IN EACH OF THE TITANIUM DIOXIDE BATCHES IS FROM ABOUT 0.1% TO ABOUT 0.2%, SAID TOTAL PERCENTAGE OF ALL SAID ELEMENTS IN EACH OF THE TITANIUM DIOXIDE BATCHES HAVING SUBSTANTIALLY THE 